1. Technical Field
This disclosure relates to electric motors and, more particularly, to sensing the initial position of the rotor of a brushless DC motor while the rotor is stationary.
2. Discussion of Related Art
A basic three-phase brushless DC(BLDC) motor includes a permanent magnet rotor and three stator windings. The windings are energized with appropriate currents and phase relationships to produce a rotating field which causes rotation of the rotor. In a typical configuration, the motor windings are energized with a 120° phase relationship between the currents of the three windings.
In order to obtain rotation of the rotor, the stator windings are energized and de-energized in a sequence that relates to the current position of the rotor. Accordingly, the motor position is sensed during operation, and the sensed motor position is used to control a commutation sequence for energizing the stator windings. In one approach, pulse-width-modulated (PWM) signals are applied to the motor windings according to the sensed motor position.
Known techniques for sensing motor position include mounting position sensors within the motor. Other known techniques include measurement of back electromotive force (BEMF) but this requires the rotor to be rotating at sufficiently high speed in order for the BEMF to be detectable. Before we can even start rotating the rotor, it is crucial to determine its initial position so that the correct first phase is energized to avoid back rotation or stalling of the rotor. One widely used technique to determine the initial position is known as inductive sense. However, inductive sense accuracy is affected by the mismatch between the phases of the motor coils in terms of resistance and inductance and the current limit measurement apparatus.
Manufacturing BLDC motors and/or measurement apparatus with tight tolerance is both high in cost and complexity. Accordingly, there is a need for improved methods that are more tolerant to these manufacturing defects.